This study was initiated to quantify the effect of different degrees of irregularity on structures designed for earthquake using simplified analysis. The types of irregularity considered were: (a) Vertical Irregularity • Mass • Stiffness -Strength (b) Horizontal (Plan) Irregularity • Diaphragm Flexibility Simple models were used to allow many analyses to be conducted in a relatively short time. For vertical irregularity studies, simple shear-type structures were designed according to the New Zealand design Standard, NZS1170.5, firstly as regular structures, and then they were redesigned as irregular structures to the same target drift. Both regular and irregular structures were then subjected to a suite of records, and vertical irregularity effects evaluated from the difference in response. For the flexible diaphragm effect study, simple models of structures were developed with: (a) a rigid diaphragm assumption; and (b) a flexible diaphragm assumption. Flexible diaphragm effects were evaluated by conducting time-history analyses and comparing the responses of structures with rigid and flexible diaphragms. A mechanics based approach was developed to quantify flexible diaphragm effects, which was shown to produce consistent results with those from time-history analyses. Relationships between the degree of irregularity and the change in behaviour were developed. This information facilitates designers and plan checkers to rapidly evaluate the likely effect of irregularity on structures. It provides guidance as to: (a) when the effect of structural irregularity can be ignored, and (b) the change in demands for different degrees of structural irregularity. The relations developed also provide a rigorous technical basis for future regularity provisions in the NZS1170.5 and other world-wide seismic design codes.